Sudhikumar AV, Mathew JM, Sunish E & Sebastian PA - European ...

Materials and Methods
182
EuropEan arachnology 2005
Study Area: Kuttanad is rightly called one of the “Rice Bowls of Kerala”, contributing nearly
20% of the total rice production of the Kerala state of India. This wetland rice agroecosystem
extends from 9° 17’ – 9° 40’ N and 76° 19’ – 76° 33’ E. It is a low lying area of costal Kerala
situated 0.6 – 2.2 m below mean sea level and formed by the confluence of 4 major river systems
viz., Meenachil, Manimala, Pamba and Achancoil draining into the Vembanad Lake. It measures
approximately 25 km east to west and 60 km north to south on the west coast of Kerala, in which
about 53,639 hectares are used for rice cultivation. This is a warm, humid region and the seasonal
variation in the temperature ranges from 21°C – 38°C. Humidity is also showing seasonal fluctuation
and the average annual rainfall received is around 300 cm of which about 83% is received
during monsoon period, from June to October.
Study Period: The investigation was carried out for a period of 2 years from June 2001 to
February 2003. Sampling was conducted in four seasons; Rabi 1 (June 2001 – September 2001),
Kharif 1 (November 2001 – February 2002), Rabi 2 (June 2002 – September 2002) and Kharif 2
(November 2002 – February 2003) at the following randomly selected 8 sites located in the same
altitude: Krishnapuram, Vellisrakka, Edathua, Champakulam, Pallikoottuma, Pallathuruthy, Nedumudy
and Moncompu. Rabi season is characterized by heavy rain (South-West Monsoon) and
high humidity. More than 80% of the total annual rainfall is received during this season. Kharif
season is characterized by low rainfall and dry weather (mEnon et al. 2000).
Sampling: Sampling was done every 15 days after transplantation (DAT) from quadrates.
Spiders were collected from 4 quadrates (1m × 1m) placed at four corners of 10m × 10m area by
visual search method between 9.30 – 11.30 hours. A sufficient core area was left to avoid edge
effects. All 4 quadrates were searched for a total of one hour. Seven visits were made per site
per season. A total of 28 quadrates were studied in each season per site. Spiders were collected
from the ground stratum and from the terminals of plants. Specimens from each quadrate were
preserved in 75% alcohol in the field and counted under a stereo-zoom microscope (Leica-MS5)
in the laboratory.
Identification of Spiders: The adult spiders were identified on species level and others on
genus or family level using available literature (tikadEr 1987, Barrion, litsingEr 1995). Monthly
data were prepared for each season with detailed information on the occurrence of mature male,
female and juvenile spiders. Voucher specimens were preserved in 75% alcohol and deposited
in a reference collection housed with the Arachnology Division, Department of Zoology, Sacred
Heart College, Cochin, Kerala, India.
Data Analysis: The diversity indices like the Shannon-Wiener index (H 1 ), which is sensitive
to changes in the abundance of rare species in a community, and the Simpson index (λ), which
is sensitive to changes in the most abundant species in a community, Margalef Richness index
(R) and Evenness index (E) of spider communities were calculated using the SPDIVERS.BAS
program of ludWig, rEynolds (1988). Shannon-Wiener index is defined as:
H 1 = -∑ i log p i
Where: p i = the observed relative abundance of a particular species (soloW 1993).
Simpson index is defined as:
λ = ∑n i (n i -1)/ [N (N-1)]
Where: n i = the number of individuals of species i , and N = ∑n i (soloW 1993) .
Margalef richness index is defined as:
R = S-1/In (n).